Frequently Asked Questions

1. Are the glazes you have in the book for oxidation or reduction at cone 6?

All of our glazes were developed in an oxidizing atmosphere--a plain old computer-controlled electric kiln. Some would probably do well in reduction, but others definitely would not because they contain significant amounts of zinc or iron. None have been tested in reduction.

2. You have both stressed the importance of slow cooling in your Clayart posts. Will I have to cool my kiln slowly to duplicate your results?

Absolutely, at least for the matte glazes. You will also need to pay attention to application thickness. You may be able to get our results with some of the glossy glazes without being quite as careful.

Slow cooling is one of the most valuable and least understood aspects of firing an electric kiln. It is the only way we know of to make attractive and durable matte glazes. Matteness in a properly formulated glaze comes from small crystals forming during cooling. Cooling rate can make a dramatic difference in the appearance of the same glaze as is illustrated below. These glazes are all Variegated Slate Blue from Mastering Cone 6 Glazes. The only difference was cooling rate and thickness of application.


The tile on the left illustrates rapid cooling. The kiln was simply turned off when peak temperature was reached. While the photo does not show it well, the glaze is glossy with only a few small crystals giving some matteness on the ridges of the texture. The center two tiles illustrate cooling at approximately the rates shown in the table below. The difference is the thickness of application with the glaze on the right tile being applied more thinly. The tile on the far right was cooled even more slowly than our current recommended rate and is a fairly dry matte.

Since publication of the book we have continued to fine tune our firing/cooling cycle to get the results we personally like. Our current cycle (which will continue to evolve) is:


* Must be adjusted for each kiln depending on thermocouple calibration

3. I don't have a computer controlled kiln. Can I slow cool?

You certainly can, although it will require a little trial and error testing. You will accomplish that more quickly if you have a thermocouple to monitor the temperature in the kiln. But try this. When your cones have bent to where you want them, turn the kiln off for about 10-15 minutes to let the temperature drop fairly rapidly. Then turn the kilns switches back on to "medium" for about 2 hours. Then turn them to "low" for another 2 hours, then Off. Keep good notes on what you did and the results. Adjust to taste.

4. I am getting some crawling with Licorice. What can you suggest?

Crawling can be caused by several things including 1) a too-thick application of the glaze, 2) dusty bisque, and 3) firing a glazed pot while it is still wet. If you are convinced none of these factor is causing your problem then a modification of the glaze composition may solve the problem for you. This glaze contains EPK as its source of clay. To minimize the tendency for crawling, it is sometimes better to calcine part of the EPK or substitute a ball clay for some of the it.

Below are 4 links to a modified Licorice recipe you may want to try depending on your specific needs and available materials. None have been tested; however, they all are fairly small modifications that should move you in the right direction. Let us know if they seem to solve your problem.

Mod 1. Try this if you have a batch already mixed that is crawling and you want to 'rescue' it by adding an equal amount of a glaze that should fix the problem.

Mod 2. Try this modification to mix a new batch using a mixture of EPK and calcined EPK (you can easily calcine you own EPK by putting some in a bisque, unglazed, open-form pot and including it in your next bisque firing).

Mod 3. This is the modification to use if you want to try a mixture of EPK and OM-4 ball clay.

Mod 4. If you prefer to work with ball clay instead of EPK try this version.

5. I work at cone 10. Will this book be of any use to me?

The answer is a definite maybe. We would make the following points:

• If you are only interested in glaze recipes you should not buy the book. All the recipes are specific to cone 6.

• If you are interested in testing your glazes to establish whether or not they are suitable for their intended use, this book is as applicable to cone 10 as it is to cone 6. And there are lots of bad glazes at cone 10 that need to be weeded out from use on functional pottery and we give a lot of information on this subject.

• If you are interested in learning the principles of formulating stable or durable glazes, this book would also be useful to cone 10 potters. While the data were developed at cone 6 we have seen nothing to indicate that the same 4 "rules" for making stable glazes would not be just as applicable at cone 10.

• If you are interested in learning how to adjust glazes for better fit with your clay this book is also of interest to you. While the specific test glazes we have developed are for cone 6, the methodology and the principles are equally applicable to cone 10.

• Will we write another book that is 100% devoted to Cone 10? We have no idea, but at least not for 2 or more years. We are still recovering from writing this one and it takes a lot of research to develop all the glazes and data. In the meantime, this book does provide a significant amount of information you will not find anywhere else that we are aware of.

6. I am having trouble with glazes like Bone and Spearmint turning out a yucky yellowish and yellowish-green respectively. Any ideas?

Yes. Several people have had this problem. In every case so far we have traced it to rutile that is different from ours or, in the case of Spearmint, to using a non-white clay body. Try replacing the 6% rutile with about 5% titanium dioxide (rutile is an ore from whiich TiO2 is extracted) and your colors will probably be closer to ours.

7. Will the book be available at my local "big-box" bookstore or on amazon.com?

The republished color version is available on Amazon and Barnes & Noble. Whether Barnes & Noble will stock it  in their stores or just sell it from their website, we don’t know

8. Do you use frits in your glaze recipes? I live in Europe and have difficulty obtaining frits that are available in North America.

Yes, we source all of the boron from frits. In our opinion, Gerstley Borate and Colemanite are too variable in composition and of questionable future availability. Below is a table of the frits we use (first column) and near-equivalents that may be more readily available in your part of the world.


If you use any of these "near-equivalent" frits you may have to adjust the other materials slightly to keep the unity formula of the glaze constant.

© 2013 John Hesselberth and Ron Roy, all rights reserved.